Compact portable electronic devices such as digital cameras and cell phones have come to be widely used in recent years. These electronic devices are continuously required to minimize volume and have light weight, and the batteries installed therein are required to realize small size, light weight and large capacity. Further, the large secondary batteries for installation in automobiles and the like are also desired to realize large secondary batteries in place of conventional lead storage batteries.
In order to respond to these requirements, there has been considerable activity in the development of secondary batteries such as lithium ion secondary batteries and alkaline secondary batteries, for example, in the development of a mixture ink used to form an electrode. Further, there has been interest in a composition for forming an underlayer which is used to form an underlayer of a mixture layer.
As important characteristics which are required for the mixture ink used to form an electrode or the composition for forming an underlayer, the uniformity in proper dispersion of an active material or a conductive assistant is listed. This is because the dispersion state of the active material or the conductive assistant in the mixture ink or the dispersion state of the conductive assistant in the composition for forming an underlayer is associated with the dispersion state of the active material or the conductive assistant in the mixture layer or the dispersion state of the conductive assistant in the underlayer, influences electrode physical properties, and eventually influences the battery performance.
Therefore, dispersion of the active material or the conductive assistant is an important issue. Carbon materials having superior conductivity (conductive assistants) are difficult to uniformly mix and disperse in a mixture ink or a composition for forming an underlayer due their large structure and specific surface area resulting in strong cohesive force. When the control of dispersibility and particle size of a carbon material as a conductive assistant is insufficient, electrode internal resistance is not decreased due to lack of formation of a uniform conductive network. As a result, a problem not to get sufficient performance of electrode materials occurs.
If dispersion of active materials in not only the conductive assistant but also in the mixture ink is insufficient, partial cohesion occurs in a mixture layer formed from the mixture ink. Then, resistance is distributed on the electrode by the partial cohesion. Thus, concentration of current occurs during use as a battery, resulting in promotion of partial heating and deterioration.
Further, it is required that the mixture ink or the composition for forming an underlayer has proper fluidity to apply onto the surface of a metal foil which functions as a current collector. Furthermore, it is required that the mixture ink or the composition for forming an underlayer has proper viscosity to form a mixture layer and an underlayer which have a surface as smooth as possible and a uniform thickness.
The mixture layer formed from the mixture ink or the underlayer formed from the composition for forming an underlayer is formed. Then, each of the layers (as a metal foil substrate) is cut out or punched into a section with a desired size and shape. Then, it is required that the mixture layer or the underlayer has a hardness that is not damaged by performing cutting or punch processing and a softness that is not broken or peeled off.
Patent Documents 1 to 4 disclose that a conductive material is mixed with an active material, this mixture is kneaded with a cellulose thickener aqueous solution, an aqueous binder such as polyethylene tetrafluoride or a latex system is added thereto, and further the mixture is kneaded to prepare a mixture ink. However, the mixture ink is in an insufficient dispersion state and is lack of flexibility. Since a desired electrode cannot be produced, good battery performance is not obtained.
In order to solve these problems, a method of using a dispersant in addition to conventional materials at the time of producing the mixture ink has been developed (refer to Patent Document 5). However, good dispersion state of the mixture ink is insufficient in the use of the dispersant. Desired electrodes and secondary batteries are not obtained in many cases. Particularly, there is a need for a mixture ink in which the dispersibility of the conductive assistant is more uniform.